What caused this to happen?

I've been shooting 16mm tri-x with a bolex and sending the film off for processing. I just got some footage back and every new take the footage goes out of focus for a split second and immediately corrects itself. I know I wasn't focusing during shooting so I am led to wonder how this could happen. I've never seen 16mm film look like it has auto-focus on but it does every new take of this footage.

And the footage I had processed before this sometimes is in focus on the left side of the frame but a little fuzzy on the right.

It sounds like the film isn't being held in place through the gate. I'm not familiar with the bolex inner-workings but check the pressure plate or the mechanics around the gate. The film should be tight and straight as it moves through. if not, it will look blurry like you describe. That's all i can think of.

a cause for this could be that the pressure plate wasn't in its forward position. on the bolex you can pull back the pressure plate to clean it. when shooting with a bolex i always check that it's in the forward position after loading. i hope this helps. sinisa.

It could be the claw. The Bolex has a built-in loop resetter which uses a spring-loaded gear that grabs and resets the lost loop. The first few frames will look just as you described while the loop is being reset. Some reasons as to why the claw is malfunctioning could be wear, off-lateral alignment, or lack of cleaning and lubing.

The film door wont let you close it if the pressure plate isn't in place ...

Is it a super16 bolex ? the extra 2mm that has ben removed from the gate leaves only a snippet of a ridge for the film to seat itself on properly and if the spring guides on the side aren't quite in the right place your film could be falling off the ridge and therefore be on an incorrect angle with regards to the film plane ...

anyhoo: get some scrap film and make a loop out of it, even if you dont have a splicer you can make one with some accurate snipping and taping - then run it though waiting and watching to see what is going on.